Control system for regulating the current in a magnetron tube



Jan. 31, 1967 L. BLOK ETAL' 3,302,060

- CONTROL SYSTEM FOR REGULATING THE CURRENT IN A MAGNETRON TUBE v Filed July 6, 1964 2 Sheets-Sheet 1 MAGNETRON ZENER 18 g; 1110! m1o3oE 19 A 6 /RESTIFIER l 37 22 20 h A To FIG.3 T

LOURENS BLOK INVENTORJ JOHANNES FIN-VAN DIJK JOHAN TH.KOTS

AGENT Jan. 31, 1967 BLOK ETAL' 3,302,060

CONTROL SYSTEM FOR REGULATING THE CURRENT IN A MAGNETRON TUBE Filed July 6, 1964 2 Sheets-Sheet z 2Q 2a 31 1 I (I r 26 Q 30A) 4, g 30 b l W '1 f l l 6 31 32 FIG.2

LOURENS BLOK INVENTORJ JOHANNES EMVAN DIJK JOHAN TH. KOTS United States Patent 3,302,060 CONTROL SYSTEM FOR REGULATING THE CURRENT IN A MAGNETRON TUBE Lourens Blok, Johannes Frederik Maria van Dijk, and Johan Theo Kots, all of Emmasingel, Eindhoven, Netherlands, assignors to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed July 6, 1964, Ser. No. 380,374 Claims priority, application Netherlands, July 17, 1963, 295,449 8 Claims. (Cl. 315307) The invention relates to a magnetron generator, particularly for high-frequency electric heating and to a magnetron for use in such a generator.

In order to reduce the power variations of a magnetron generator which may occur, for example, because of fluctuations of the supply voltage, the supply circuit of the magnetron generator includes a control-circuit of a control-device. The output circuit of the control-device is connected to a magnet coil of the magnet system of the magnetron. In operation, the control-device governs the energizing current through the magnet coil so as to stabilize the magnetron direct current. Furthermore, for intermittent operation of the magnetron, the supply circuit includes acontrol-switch for switching the magnetron on and 0E.

The invention has for an object to provide in such a magnetron generator a simplification of the magnetron structure and a particularly simple control-device wherein the reliability of operation and the lifetime of the magnetron are improved.

In accordance with the invention, the magnet system of the magnetron comprises an electromagnet having a magnet coil constructed as a separate control-coil and that of the magnet coil. The control-device further comprises a permanent magnet, the magnetic field of which supports an auxiliary control-circuit which, when the magnetron is switched on by means of the control-switch, supplies an auxiliary energizing current decreasing with time for the magnet coil constructed as a control-coil.

The invention and its advantages will now be described more fully with reference to the figures.

FIG. 1 shows a detail diagram of a magnetron generator according to the invention.

FIG. 2 shows the construction of a magnetron according to the invention and FIG. 3 shows a, time diagram for explaining the operation of the magnetron generator shown in FIG. 1.

The magnetron generator shown in FIG. 1 comprises a magnetron 1 of, for example, 2 kw. for producing ultra high-frequency oscillations having .a Wavelength of 12 cms. The anode and the cathode of the magnetron 1 are connected to the output terminals of a rectifier 2, which is in turn connected to the terminals of a source of threephase alternating-voltage. The magnetron generator forms part of .a high-frequency heating furnace (not shown). For intermittent operation, the supply circuit includes a control-switch 3 for switching the magnetron on and off.

In order to reduce variations of the output power of the magnetron 1 produced, for example, by supply voltage fluctuations, the magnetron generator shown comprises a controldevice 4 comprising a control-circuit 5 connected to the direct-current supply circuit of the magnetron 1. The output circuit of the control-device 4 is connected to a field coil 6 of the magnet system of the magnetron 1. The energizing current of the field coil, and hence the magnetic field of the magnetron 1, is governed by the control device 4. In order to obtain a particularly simple and eflicient magnetron structure with this control-device t, the magnet system of the magnetron 1 is formed, as will be explained more fully with reference to FIG. 2, by the magnet coil 6, constructed as a separate control-coil, together with a permanent magnet 7, shown diagrammatically in FIG. 1, the magnetic fields of said members supporting each other.

In the embodiment shown, the control-circuit 5 of the control-device 4 is formed by a series resistor 8 included in the DC. supply circuit of the magnetron 1. Coupled to resistor 8 is a smoothing filter for smoothing ripple voltages, comprising a series resistor 9 and a capacitor 10. The capacitor is connected through a comparatively high series resistor 11 to the base electrode of a transistor 12 connected as a comparison device and including a Zener diode 13 in its emitter circuit. The base electrode of the transistor 12 is connected through a base-emitter resistor 14 to the emitter electrode. The supply voltage of the transistor 12 is derived from a full-wave rectifier comprising diodes 16, 17 connected via a transformer 15 to the AC. supply and a smoothing capacitor 18, connected to the output circuit of the rectifier.

When, in the device so far described, the voltage of the capacitor 10 exceeds the Zener voltage of the Zener diode 13, the emitter circuit of the transistor 12 is traversed by an emitter current, the value of which is proportional to the difference in the voltage across the smoothing capacitor 10 and the Zener voltage. The value of the emitter current is substantially independent of the temperature over a considerable temperature range since the base-emitter voltage of the transistor 12 and the Zener voltage have temperature coefficients of opposite polarities.

For controlling the energizing current of the magnet coil 6, the emitter current of the transistor 12 is amplified in a two-stage D.C. amplifier comprising transistors 19, 20. The amplified emitter current of the transistor 12 is supplied to the magnet coil 6 included in the collector circuit of the transistor 20. The transistor 19 comprises in its emitter circuit a feedback resistor 21 and the transistors 19, 20 are provided with stabilizing resistors 22, 23, respectively, connected to the base electrodes.

The control-device 4 described above provides, Iby stabilisation of the magnetron direct current, an effective reduction of variations occurring in the power supplied by the magnetron 1. If the magnetron direct current increases, the increase in the voltage of the capacitor 10 will produce an increase in the emitter current of the transistor 12. In turn, the energizing current through the magnet coil 6 increases, resulting in an increase in the magnetic field of the magnetron 1, the latter increase counteracting the increase in the magnetron direct current.

Conversely, a decrease in the magnetron direct current causes a decrease in the magnetic field which, in turn, counteracts the decrease of the magnetron direct current. Variations of the power supplied by the magnetron, which may amount to many hundreds of watts, are reduced to a few tens of watts.

On the one hand this control provides a reduction of variations of the output power of the magnetron of many hundreds of watts by means of a control-device 4 adapted to provide a power of 30 watts, so that the control-device 4 is particularly simple. Furthermore, it is even possible to equip the control-device with transistors, as is indicated in the embodiment shown, whereas on the other hand the structure of the magnetron 1 is considerably simplified.

FIG. 2 shows in detail the structure of the magnetron 1. The magnetron tube is provided in known manner with a control cathode 25, an anode 26 surrounding the cathode, resonant cavities formed by radial partitions 27, and pole pieces 28 for concentrating the magnetic field in the space between the cathode 25 and the anode 26. The

magnetron cathode is cooled by air by means of cooling vanes 29. The anode of the magnetron is cooled by water.

The magnetron tube is surrounded by a magnetic circuit comprising two permanent magnets 30, lying each on one side of the magnetron tube and consisting for example of high-grade magnet steel on the basis of aluminum-cobalt-nickel, if desired with titanium the so-called ticonal or else ceramic permanent magnetic material with a non-cubic crystal structure for example consisting of polyoxides of barium and iron with hexagonal crystal structure (so-called Ferroxdure (magnadur) The magnetic field produced by the permanent magnets 30 is guided by means of a soft-iron magnet yoke 31 towards the pole pieces 28 in the magnetron tube. Apart from the permanent magnets 30 the magnetron comprises a magnet coil 6 constructed as a separate control-coil which is connected to the output circuit of the control-device 4. The magnet circuit further comprises a soft-iron core 32 which is arranged in a radial direction with respect to the magnetron tube and supplies, upon energisation, a magnetic field through the magnetron tube in the same direction as the magnetic field of the two permanent magnets 30.

Since the magnetic field of the magnet coil 6 supports that of the permanent magnets 30, the latter need supply only a fraction of the required magnetic field, so that smaller permanent magnets 30 may suifice. Furthermore, with the control-device 4 shown, demagnetization phenomena need not be feared since the magnetic field of the magnet coil 6 always has the same direction as that of the permanent magnets 30. With the magnetron 1 shown an economy of about 20% of the material of the permanent magnets 30 was obtained.

On the other hand the axial arrangement of the magnet coil 6 with respect to the magnetron 1 ensures that substantially the Whole field of the magnet coil 6 is guided through the magnetron tube, in contrast to a different arrangement of the magnet coil 6. If, for example, the magnet coil 6 were arranged to surround the permanent magnets 30, the field of the magnet coil 6 would extend for the major part as a leakage field beyond the magnetron tube owing to the low permeability of the permanent magnetic material (,u abou-t 4).

In accordance with the invention, a simple and rugged structure of the magnetron 1 is achieved wherein a magnet coil 6 of small size and a comparatively small number of turns may suffice. At the same time, a very high control-sensitivity is obtained so that variations of the output power of the magnetron 1 of many hundreds of watts may be reduced to acceptable limits by means of a control-device of 30 watts.

In operation the device so far described provided excellent results, but from elaborate experiments it was found that the lifetime of the magnetron 1 did not come I up to the expectations, which had to be ascribed to a transient peak current occurring across the magnetron 1 when switched on and producing an overload thereof. At the instant the magnetron 1 is switched on by the control-switch 3, the magnetic field of the magnetron is supplied only by the permanent magnets 30. The permanent magnet-s supply only a fraction of the required magnetic field so that initially the magnetron direct current has a maximum value until, after a time lag determined by the time constant of the control-device 4, the magnetic field of the magnetron 1 is brought to its normal value by the energisation of the magnet coil 6. The time constant of the control-device 4 in the embodiment shown is mainly determined by the time constant of the smoothing filter 9, 16 and the value of the magnet coil 6.

In the time diagram shown in FIG. 3, the magnetron direct current has the variation indicated by curve a. At the switching-on instant T the magnetron direct current has a maximum value (for example three times its normal value). The control-device 4 becomes operative after a time lag determined by the time constant of the control-device, at which time the magnetron direct cu rent is brought to its normal value I Whilst the advantages obtained are maintained, said difiiculty is obviated in accordance with the invention iia simple manner by providing the control-device 4 (of FIG. 1) with an auxiliary control-circuit 33, which supplies, when the magnetron 1 is switched on, an auxiliary energizing current decreasing with delay for the magnet coil 6 constructed as a control-coil. In the embodiment shown the auxiliary control-circuit 33 is formed by a capacitor 35 connected through a switching contact 34 to the output circuit of the rectifier 16, 17 and connected by means of a series resistor 36 through a second pair of input terminals 37, 38 to the input circuit of the transistor amplifier 19. Contact 34 is coupled :to the switch 3 so that when the control-switch 3 is open, the switching contact 34 is closed and the capacitor 35 is charged to the full supply voltage of the rectifier 16, 17. Through the series resistor 36 an auxiliary control-current passes to the base electrode of the transistor 19, said current producing, subsequent to amplification in the transistors 19, 20, a maximum energizing current through the magnet coil 6. Then the magnetic field of the magnetron 1 composed of the magnetic field of the permanent magnet 7 and the magnetic field of the magnet coil 6- has a maximum value.

When the control-switch 3 is closed at the instant T the switching contact 34 is opened at this instant, whereupon the capacitor 35, which is charged to the full output voltage of the rectifier 16, 17, is discharged through the series resistor 36, so that an auxiliary energizing current decreasing with a delay is produced through the magnet coil 6 via the transistors 19, 20. The rate of decrease of said current is determined by the discharge time constant of the capacitor 35, which is mainly determined by the value of the capacitor 35 and the value of the resistor 36 connected in series therewith. Thus, from the switching-ion instant T the magnetic field of the magnetron will decrease gradually from a maximum value, so that the magnetron direct current will gradually increase until by the operation of the control-device 4 the magnetron direct current is stabilised on its desired value via the control-circuit 5.

From this instant the control exercised by the auxiliary circuit 33 is taken over by the control-circuit 5 and the auxiliary control-circuit 33 no longer affects the control-process. In order to obtain a smooth control without transient phenomena, it is advantageous to provide the auxiliary control-circuit 33 with a time constant which is greater than the time constant of the overall control-device 4. In the embodiment shown, the time constant of the auxiliary control-circuit, which is determined by the time constant of capacitor 35 and resistor 36, is a factor 7 times greater than that of the overall control-device.

When the control-switch 3 is opened, in this state of operation the switching contact 34 is again closed and the capacitor 35 of the auxiliary control-circuit 33 is charged to the full output voltage of the rectifier 16, 17, after which at the next-following closure of the control-switch 3, the cycle described above is repeated. It is advantageous to connect the auxiliary controlcircuit 33 to the input circuit of the DC. amplifier 19, 20, since the capacitor 35 need furnish only a low discharging current in order to cause the full energizing current to pass via the amplifiers 19, 20 through the magnet coil 6, so that a comparatively small capacitor 35 may sufiice.

The time diagram of FIG. 3 illustrates the magnetron direct current at the closure of the control-switch 3. In accordance with the invention, the magnetron direct current follows the path indicated by the curve b. As can be seen from the figure, the magnetron direct current gradually rises from a minimum value at the switchingn instant T to the desired value. Peak currents through the magnetron 1, which would shorten the useful life of the magnetron 1, as stated above, do not appear in the device according to the invention. Instead, when the magnetron is switched on direct current will gradually rise to the desired value, which results in a prolongation of the lifetime of the magnetron 1 in intermittent operation.

Not only a particularly high sensitivity and reliability of operation, but also a simplification and a prolongation of the lifetime of the magnetron structure are obtained by using the device. according to the invention, which is therefore particularly interesting for practical use.

A practical, elaborately tested embodiment of the device according to the invention had the following data:

Overall size of the magnetron 1 mms 130 X 80 X 130 Power of the magnetron 1 kw 2 Control-device Transistors 12, 19 OC74 Transistor 20 ASZlS Resistor 8 ohms Resistor 9 do 680 Capacitor 10 .f 1000 Resistor 11 ohms 1500 Zener diode 13 OAZ206 Capacitor 35 ;tf 500 Resistor 36 ohms 10,000

It should be noted here that the central-device described above may also be constructed in a different manner. The switching contact 34 might be constructed so that, when the control-switch 3 is actuated, the switching contact 34 is closed prior to the switching-on instant, thereby charging the capacitor 35, whereas at the switching-on instant the switching contact 34 is opened like in the device shown. All of these embodiments have the feature that when the magnetron generator 1 is switched on, an auxiliary energizing current which decreases with time lag passes through the magnet coil 6.

What is claimed is:

1. A control system for regulating the current in a magnetron tube comprising, means for establishing a magnetic field in said tube comprising a permanent magnet and an electromagnet having a field coil, voltage supply means for energizing said tube and said field coil, first control means for varying the current in said field coil as a function of the magnetron tube current, said first control means having a given time constant which determines its response time, switching means for apply- 69 ing the energy of said voltage'supply means to said magnetron tube, and second control means responsive 6 to the closure of said switching means for supplying to said field coil a gradually decreasing current.

2. A system as described in claim 1 wherein said second control means has a time constant which is greater than the time constant of said first control means so that upon energization of said tube the rate of buildup of the current supplied to said field coil by said first control means is greater than the rate of decrease of the current supplied to said field coil by said second control means.

3. A system as described in claim 2 wherein said electromagnet is arranged to produce a magnetic field which supports the magnetic field of said permanent magnet.

4. A system as described in claim 1 wherein said switching means further comprises a switch contact arranged to open upon the application of said supply energy to said magnetron tube by said switching means, and wherein said second control means comprises a capacitor, a source of direct voltage, a charge path including said switch contact interconnecting said direct voltage source and said capacitor, and a discharge path for said capacitor which includes resistance means.

5. A system as described in claim 4 wherein the time constant of said capacitor discharge path is greater than the time constant of said first control means.

6. A system as described in claim 5 wherein said discharge path does not include said switch contact.

7. A system as described in claim 1 wherein said first control means comprises a resistor connected in series with the magnetron tube, a low pass filter across said resistor, a transistor having a base electrode and an emitter electrode, a Zener diode connected in series with said emitter, a second resistor connected in series between the output of said filter and said base electrode, a transistor direct current amplifier having an input circuit and an output circuit and means for coupling said output circuit to said field coil to supply current thereto, means for coupling said emitter electrode to said input circuit so that the transistor emitter current is supplied to said direct current amplifier for controlling the current supplied to said field coil, and means for coupling the output of said second control means to the input circuit of said direct current amplifier.

8. A system as described in claim 1 wherein said second control means includes an energy storage element which has a time constant greater than said given time constant whereby the current supplied to said field coil by said second control means decreases in an exponential manner.

References Cited by the Examiner UNITED STATES PATENTS 2,296,764 9/1942 Braden 331-438 2,450,629 10/1948 Bondley 3l539.71 X 2,523,684 9/1950 Dow 331-88 2,648,772 8/1953 Dawson et al. 33186 2,979,671 4/1361 Schall 331-88 X FOREIGN PATENTS 717,001 10/1954 Great Britain.

HERMAN KARL SAALBACH, Primary Examiner. P. L. GENSLER, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,302,060 January 51, 1967 Lourens Blok et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that. the said Letters Patent should read as corrected below.

Column, 1, lines 35 to 41, strike out "net coil constructed as a separate control-coil and that of the magnet coil. The control-device furthercomprises a permanent magnet, the magnetic field of which supports an auxiliary control-circuit which, when the magnetron is switched on by means of the control-switch, supplies an auxiliary energizing current decreasing with time. for the magnet coil constructed as a control-coil." and insert instead net coil constructed as a separate control-coil and a permanent magnet, the magnetic field of which supports that of the magnet coil. The controldevice further comprises an auxiliary control-circuit which, when the magnetron is switched on by means of the controlswitch, supplies an auxiliary energizing current decreasing with time for, the magnet coil constructed as a control-coil. column 5, line 40, for "central" read control Signed and sealed this 17th day of, October 1967.

(SEAL) Attest:

EDWARD J. BRENNER Commissioner of Patents EDWARD M.FLETCHER,JR. Attesting Officer 

1. A CONTROL SYSTEM FOR REGULATING THE CURRENT IN A MAGNETRON TUBE COMPRISING, MEANS FOR ESTABLISHING A MAGNETIC FIELD IN SAID TUBE COMPRISING A PERMANENT MAGNET AND AN ELECTROMAGNET HAVING A FIELD COIL, VOLTAGE SUPPLY MEANS FOR ENERGIZING SAID TUBE AND SAID FIELD COIL, FIRST CONTROL MEANS FOR VARYING THE CURRENT IN SAID FIELD COIL AS A FUNCTION OF THE MAGNETRON TUBE CURRENT, SAID FIRST CONTROL MEANS HAVING A GIVEN TIME CONSTANT WHICH DETERMINES ITS RESPONSE TIME, SWITCHING MEANS FOR APPLYING THE ENERGY OF SAID VOLTAGE SUPPLY MEANS TO SAID MAGNETRON TUBE, AND SECOND CONTROL MEANS RESPONSIVE TO THE CLOSURE OF SAID SWITCHING MEANS FOR SUPPLYING TO SAID FIELD COIL A GRADUALLY DECREASING CURRENT. 